Engineering sustainable food systems: a review of rotary drying technologies for the valorisation of heterogeneous agri-food waste into animal feed

Agri-food systems generate large volumes of wet organic waste that pose persistent sanitation and environmental challenges while simultaneously embodying untapped nutritional value for livestock feeding. In Ghana, municipal solid waste streams are dominated by biodegradable food residues with high moisture content, leading to rapid spoilage, elevated disposal costs, and greenhouse gas emissions, alongside rising feed costs and dependence on imported feed ingredients. This review examines sustainable drying technologies for converting heterogeneous agri-food waste into animal feed, with particular emphasis on rotary dryers as a robust engineering solution. The paper synthesises evidence on drying technology categorisation, rotary dryer configurations, and the fundamental heat and mass transfer mechanisms governing solids transport, residence time, and moisture removal. Design variables, including drum inclination, rotational speed, and internal flight geometry, are critically reviewed in relation to throughput, thermal efficiency, and tolerance for feedstock heterogeneity. Comparative analysis with tray and fluidised bed dryers highlights the suitability of rotary systems for mixed, sticky waste streams typical of urban markets and food service operations. Beyond process performance, the review explicitly links drying conditions to nutritional outcomes, focusing on crude protein retention and metabolizable energy preservation. Reported findings indicate that rotary dryers, when operated within moderate temperature and residence-time windows, can stabilise food waste while retaining a substantial proportion of feed value. Barriers to adoption, including capital costs, maintenance requirements, infrastructure gaps, and regulatory uncertainty, are discussed alongside pathways for context-appropriate deployment. The study concludes that modular rotary dryers integrated with solar–biomass hybrid energy systems and embedded within circular bio-economy hubs offer a viable pathway to bridge waste management and livestock productivity, contributing to resilient and sustainable food systems in Ghana.